Texas Two Step Taco

John Murray

Been Around the Block
More wacky fuel system pics for you, Pat. First pic shows the solenoid valve that shuts off the gas supply from the small gas tank under the carb. A positive fuel shutoff is a rules requirement. Fuel from the solenoid goes to the two needle valves on the other side.
The tank is positioned so the top is about 20mm below the height of the idle discharge holes. No float bowl required so it's very simple.
solenoid.jpg


Some carbs allow a separate feed for the idle discharge holes and the intermediate transfer slots. This is ideal as it allows independant tuning of the idle and off-idle up to about 1/4 throttle or so. That's what these two needle valves are for, one for idle, the other for the area in between idle and the startup of the main circuit. It's possible to get a very sharp and clean transition with this.
rcneedles.jpg

The needle valves are remote valves made for RC cars and boats. They give a nice fine adjustment and stand up to alky and nitro. A nice feature is the arm that gives a coarse adjustment when it's turned. This is an easy way of adding enrichment for cold starts without altering the fine adjustment. You can see the arm in the pic below.
aaa.jpg

The gas is a very small proportion of the total fuel flow but it helps get a little heat in the engine. The main (alky) supply runs from the tank shutoff through a pill and pill holder (to set the max flow) then through a throttle actuated barrel valve to the carb nozzles (two barrel carb). So again, there's no float bowl and it's a simple and rugged system. Pill changes can be made easily in maybe half a minute without spilling much fuel at all.
 
Last edited:

Texasstar

Can't is a four letter dirty word
You could. Or you could just learn t... I.. I can't bring myself to say it! Help me Teazer!!

I have my Ricky Gadsen YouTube how to launch patch. Oh I can load the clutch but have you seen what these guys have to get to a sub 1.50 60’. Sliders, two steps, delay boxes, override transmissions, et al. It’s all about the clutch. Lol! What did you think about the engineering? Basically a lock out on the clutch.


Sent from my iPhone using Tapatalk
 

John Murray

Been Around the Block
I think the appeal of the rekluse is that they're a simple drop in install. But I'd be a bit concerned that the thickness of it would prevent the use of enough plates to hold firmly. I guess you could make a custom basket to suit an off-the-shelf rekluse along with a sufficient number of off-the-shelf plates. My son Kurt had one in a KX250F; seemed to work well enough.

Those guys with all the launch gadgets are running bikes with horsepower in the hundreds aren't they? I think it's overkill for a simple (but fast) Bul.
 

Texasstar

Can't is a four letter dirty word
I think the appeal of the rekluse is that they're a simple drop in install. But I'd be a bit concerned that the thickness of it would prevent the use of enough plates to hold firmly. I guess you could make a custom basket to suit an off-the-shelf rekluse along with a sufficient number of off-the-shelf plates. My son Kurt had one in a KX250F; seemed to work well enough.

Those guys with all the launch gadgets are running bikes with horsepower in the hundreds aren't they? I think it's overkill for a simple (but fast) Bul.

Agreed. I am eating more spinach. Maybe one day I will do the slave cylinder conversion ;)
IMG_9892.jpg



Sent from my iPhone using Tapatalk
 

Texasstar

Can't is a four letter dirty word
More wacky fuel system pics for you, Pat. First pic shows the solenoid valve that shuts off the gas supply from the small gas tank under the carb. A positive fuel shutoff is a rules requirement. Fuel from the solenoid goes to the two needle valves on the other side.
The tank is positioned so the top is about 20mm below the height of the idle discharge holes. No float bowl required so it's very simple.
View attachment 235550

Some carbs allow a separate feed for the idle discharge holes and the intermediate transfer slots. This is ideal as it allows independant tuning of the idle and off-idle up to about 1/4 throttle or so. That's what these two needle valves are for, one for idle, the other for the area in between idle and the startup of the main circuit. It's possible to get a very sharp and clean transition with this.
View attachment 235551
The needle valves are remote valves made for RC cars and boats. They give a nice fine adjustment and stand up to alky and nitro. A nice feature is the arm that gives a coarse adjustment when it's turned. This is an easy way of adding enrichment for cold starts without altering the fine adjustment. You can see the arm in the pic below.
View attachment 235552
The gas is a very small proportion of the total fuel flow but it helps get a little heat in the engine. The main (alky) supply runs from the tank shutoff through a pill and pill holder (to set the max flow) then through a throttle actuated barrel valve to the carb nozzles (two barrel carb). So again, there's no float bowl and it's a simple and rugged system. Pill changes can be made easily in maybe half a minute without spilling much fuel at all.

Nitro now we are talkin! Can we put the tank where I had the tank the first time?
IMG_9891.jpg



Sent from my iPhone using Tapatalk
 

John Murray

Been Around the Block
The tank can go anywhere if you use a pump. Make sure it's somewhere where it's easy to fill though; you're gonna be topping it up an awful lot on the dyno..
 

Texasstar

Can't is a four letter dirty word
What percentage of blowdown were the old Bultacos and how do we calculate it?
I have been watching the chainsaw builders and they run a higher percentage of blowdown in their bigger saws for more power.

Sent from my iPhone using Tapatalk
 

John Murray

Been Around the Block
Basically you measure and calculate blowdown Time/Area and or Specific Time Area in exactly the same way you would normally, but in this case just measuring the area above the top of the transfers. What you do with this data is the next issue, and I really really think it should be handled by an up-to-date, proven sim - and by that I mean EngMod2T and not something like MOTA etc etc. If you just want a simple tool to calculate the blowdown STA Torqsoft have one: http://www.torqsoft.net/exhaust-time-area.html

Quoted from Frits Overmar:
Unless you keep the revs really low, a single exhaust port with a 70% chordal width does not give sufficient blowdown time.area, but because of the reason I explained above, making the port wider will be even worse (large radii). You will either have to use a bridge, add auxiliary exhaust ports, or raise the timing of the single port to well above the 190° that is optimal for pipe resonance.

The Aprilia cylinder is extreme in that respect: the middle exhaust port is 38 mm wide (older versions were 40 mm wide) and there are huge auxiliaries but even so the middle port is raised to 196° and then it gets a large radius at the top. The radius makes it difficult to measure the timing but if you shove a piston ring in the bore until you can just see light between the ring and the bore, you will find an exhaust timing of 202°.

Don't try this at home, unless you want your engine to produce its maximum power at a mean piston speed of 23.6 m/s. But then your transfer time.area will probably be insufficient. And you know: if you raise the transfers, that will eat into the blowdown time.area.

Why does Aprilia use an exhaust timing that is too high for optimum resonance? It is a compromise: any lower, and the maximum torque will rise, but the reduced blowdown time.area will cause an early torque collapse and the product of torque times revs (yes, that's power) will be lower.

STA stands for Specific Time.Area. STA depends on rpm: the higher the revs, the shorter the ports are open per revolution. So I can only give you STA numbers if you specify for what rpm you wish to know them.

It's like this: a cylinder has port windows with a certain width and certain distances from the cylinder top plane to port roof and port floor. You can only express the window dimensions in millimeters and square millimeters (or in other funny length and area units that english-speaking folk still use).

Put this cylinder on an engine with a crankshaft with a certain stroke and a certain conrod length. Now you can also express the window dimensions in crankshaft degrees.

Now we can look at the angle.area concept (the point between angle and area indicates a multiplication; I make a point of writing it like this )

Let us assume a port window is 1 mm wide. Turn the crankshaft until the window is on the verge of opening. Its open area is still zero.
Then turn the crankshaft 1° further. Let's say the piston descends 0.5 mm, so the open window area is 0.5 mm height * 1 mm width = 0.5 mm² , and it has been open for 1°; that yields an angle.area of 1° times 0.5 mm² = 0.5 °mm².

Then turn the crankshaft 1° further again. That first 0.5 mm² open area has now been open during 2°; and as the piston has descended some more, there is now some additional open window area that has been open for 1°. Multiply all those pieces of open window area with the number of crankshaft degrees they have been open, all the way from initial port opening till port closing, and you get the total angle.area of the port.
But for gas flow it does not matter during how many crank degrees an area has been open; what matters is the number of seconds it has been open.

That is where engine rpm comes into play: twice the revs means half the time; angle.area divided by rpm is time.area. And if you divide time.area by the cubic capacity of the cylinder that has to be filled (or emptied), you have specific time.area.

You may have noticed that in previous posts I sometimes talked about angle.area, and sometimes about time.area. And hopefully now you understand why. When I talk about an engine, I use angle.area. When I talk about a running engine, I use time.area.

With the angle.area values I posted, you should be able to work out the specific time.areas for 13.000 rpm. And these are universal; if you manage to get the same STA values for an engine with any cubic capacity and any rpm, you're doing fine.

With most old style engines you're likely to run into physical limitations before you reach ideal blowdown conditions, unless of course you're prepared to do some pretty intense cylinder surgery involving welding etc. You do have the advantage of having a bridged port and a much better duct shape than the angle port barrels have.

Frits touched on an important point - the transfer sizing should be follow on from the exhaust port sizing. Too much transfer is counterproductive yet often we see cylinders done by professional "porters" that have multiple transfers added yet still have insufficient exhaust. I guess it's much easier to do this than fix the exhaust, and the customer is impressed by all those new holes in his barrel. Besides, hardly any of these old Buls ever see a dyno.

And I have to apologize - between the drill rig work and renovations at home I haven't got around to sending that carb yet. I promise I'll get it away in the next few days.
 
Last edited:

Texasstar

Can't is a four letter dirty word
So I am starting at today’s technology and working backwards to understand the how and why of porting but more importantly what impacts the pressure wave coming back. I have sat watching the fuel atomize and come out my carburetor on the pipe and watch it get sucked in. Fascinating. So i went over to BB’s and he had two snowmobile engines that I could look at. One was a stock Arctic Cat 1000 with a power valve and case reeds and the other was a triple sled that had been professionally ported. I did not take pictures of it out of respect but I did stick my fingers in it and noticed and noticed the ports that run from the transfers to the intake in the reed at the bottom of the intake below the intake. I asked what are these? BB quipped those are boyeson ports and every modern day 2 stroke has them. I would love to hear all about Boyeson ports!


Sent from my iPhone using Tapatalk
 

teazer

Over 10,000 Posts
DTT BOTM WINNER
Blowdown is the time that the exhaust port is opened before the transfer ports open. If that is too small a time-area, the combustion chamber is still full of hot gas when the transfers open, so the flow is from the combustion chamber into the crankcase, until the pressure drops enough.

One way to minimize that is to open the exhaust earlier or to open the transfers later. but the other alternative is to increase the exhaust area so that the pressure can drop during the same time. OR - change the pipe so that it creates a stronger negative pressure wave. That will scavenge the cylinder more effectively.

Many/most/all of our old two strokes have insufficient port are and port timing for the sort of power we now try to extract. Conventional wisdom is that there is a limit to exhaust port width because above a certain width, the rings bulge out too far and beat themselves and the port roof to death. The solution to that dilemma is a bridged exhaust or additional ports and with a bridged exhaust it's easy to add eyebrow ports for more initial port area to facilitate that pressure drop.

So to get ideal blowdown for your application, you can change port time, port area or the pipe or all of the above. But keep in mind that whatever you calculate as ideal is only ideal at one engine speed and pipe temperature. At lower revs than peak, the pipe temperature could be lower so the pulses arrive at a different time. And of course pipes resonate at a certain speed and above and below that point, pulses arrive later or earlier.

You asked about Boyeson ports. With a cylinder mounted reed box, the gas essentially fills the crankcase and when the piston decends, it compresses the gas and that makes the gases flow up the transfer ports. In a simple model, the intake draws in air as the piston rises and gas goes up the transfer ducts as the piston descends. No need to connect the two ports.

But what if the suction wave generated by the divergent cone of the exhaust were to arrive while the transfers are open? In that case, that negative pressure wave would "suck" gas out of the crankcase and on a reed motor, it might even be able to suck gas in from the intake port. And to facilitate that, it's necessary to short circuit the flow pattern and that's where those ports came from. They also allow access to a larger volume to draw from.

It is said that some motors benefit from those ports and others do not. And given the complexity of pressure wave events, it's easy to see what that might be the case.

With crankcase reeds, all transfer ports are open to the intake duct and the large volume crankcases to take advantage of those effects.

The rhetorical question John asked about adding transfer port area when teh exhausts were still limited probably comes down to teh fact that engines were limited in all teh ports, so tuners increased exhaust ports as far as they could and then realized that the limiting factor was not transfer time-area. That wasn't easy to fix but they added more transfer time-area to the point that they had more area than they needed because the exhaust was now the limiting factor. Some tuners then started to add additional small exhaust ports which also improved blowdown.

One issue we all run into is real estate. With an existing barrel it's often impossible to get the desired port time-are without breaking into fresh air or the water jacket. On a GT750 that happens with two of the transfers fairly early on and the exhuast will also break through at relatively modest state of tune. Brett DeStoop in Australia welded and machined the heck out of a GT750 barrel until he had cast his own barrels and liners with split eyebrow exhausts and tons on transfer area plus huge double intake reeds. Most of us won't/can't reach that point..

For an 1/8th mile motor, you could machine a new barrel from billet and have the bore plated. You might have to weld up the crankcases or have Ralph S machine a set of billet cases and if you do that you can incorporate and transmission and clutch you like.
 

Texasstar

Can't is a four letter dirty word
So the boyeson ports allow the negative pressure wave to super charge the cases as the piston is going down?


Sent from my iPhone using Tapatalk
 

teazer

Over 10,000 Posts
DTT BOTM WINNER
They allow more gas to enter the combustion chamber through the wave action generated by the exhaust but not necessarily as the piston is falling. At BDC the transfers are wide open as is the exhaust. If a negative pressure wave arrives as the piston is rising, say just before the transfer closes it will cause more gas to flow from the crankcase and to draw from the intake.

The low pressure wave can be say 0.8 bar just after BDC while transfer ports are still open and it will tend to increase during the time that the piston is rising so that it's around 1.0 bar at Transfer port closing and will be a positive wave just before the exhaust port closes which is why they used to call that the stuffing wave as it stuffed some gas back into the cylinder just before the exhaust door is slammed shut.

Basically, anything that allows the motor to cram more fuel air mix into the combustion chamber is potentially beneficial. I suspect that there's more to be squeezed out of your motor but a lot more to be gained in terms of results from rider input.
 

John Murray

Been Around the Block
I realize Boyesens are very commonly used from the factory, but I seem to remember Jan Thiel once saying that he found no evidence of flow through the reed during the transfer-open period. I'll see if I can find more on his testing.

Boyesen ports seem to me to have something in common with boost bottles - there is practically no reliable dyno data anywhere showing they do anything at all when added to engines that didn't originally have them.
 

Texasstar

Can't is a four letter dirty word
I realize Boyesens are very commonly used from the factory, but I seem to remember Jan Thiel once saying that he found no evidence of flow through the reed during the transfer-open period. I'll see if I can find more on his testing.

Boyesen ports seem to me to have something in common with boost bottles - there is practically no reliable dyno data anywhere showing they do anything at all when added to engines that didn't originally have them.

Bb was telling me this story about Thiel and that there was a heated discussion on the two stroke FB page about that one day. I think Fritz was on that page. I also asked BB how that would work in a case reed engine.


Sent from my iPhone using Tapatalk
 

DTT Bike Of The Month Gallery

DTT Light or Dark

www.jadusmotorcycleparts.com
www.cognitomoto.com
https://www.townmoto.com
www.speedmotoco.com
www.lostapostle.ca/
www.sparckmoto.com
Top Bottom